Trunked RF communication systems exist to effectively allocate limited channel resources among a plurality of subscriber units. This is accomplished by reusing the same set of available RF frequencies over many different subscriber groups. A central controlling computer, with knowledge of the currently available RF frequencies receives channel allocation requests from subscriber units on an RF control channel. This computer then assigns a voice channel on a predetermined basis to the requesting subscriber unit's group of users.
In a basic RF trunked system, there exists a high degree of flexibility to partition voice conversations between different groups so that no one group of users is specifically aware when another group of users makes use of the system. Typically, these groups are divided into subgroups so that calls may be made upon either a group, subgroup or individual basis depending upon the type of communication desired by an initiating subscriber.
To establish a voice communication between a group of subscriber units operating on a trunked system, a subscriber unit transmits a data packet called an ISW (Inbound Signalling Word) on a control channel that is maintained for such purposes. The ISW contains at least the requesting subscriber unit's unique ID code, which may contain or be used to obtain the requesting subscriber's current talk-group. The request is forwarded to a TCC (Trunked Central Controller, also known as a central controller), which decodes the request and transmits on the control channel a data packet called an OSW (Outbound Signalling Word) to all subscriber units, which continuously monitor the control channel when not participating in a voice conversation. The OSW is a channel grant that contains the talk-group code of the requesting subscriber unit and the voice channel number assigned for the conversation. The OSW causes the requesting subscriber unit to move to the voice channel and commence transmitting, while simultaneously causing all other subscriber units in the same talk-group to move to the voice channel as listening units. In this way, a group call is set up. If, however, all voice channels are in use when a subscriber unit transmits an ISW, the TCC typically sends the requesting subscriber a Busy OSW.
In addition to voice messages, it is desirable to send data information across a trunked radio channel. In some data systems, a subscriber obtains a trunked data communication channel via the same procedure used to obtain a voice channel. However, this practice is inefficient and spectrally wasteful, due to the time it takes for a requesting subscriber to transmit an ISW and receive a channel grant OSW from the TCC, and the time it takes to set up and terminate a call on a voice channel. At contemporary data transmission rates, it is anticipated that an entire typical data message would take substantially less time to transmit than the time required to obtain a channel (approximately 0.5 seconds). Thus, assigning a data channel pursuant to the same procedure as assigning a voice channel would be wasteful of spectrum and consume precious system time that could be better used to transmit data messages.
Other trunked communication systems desirous to accommodate data traffic have permanently dedicated one or more channels to handling data traffic. While this avoids the access time problem noted above, this technique is contrary to the basic principles of trunked communication systems, which strive to allocate channel resources across a plurality of users as required. Therefore, the practice of having dedicated data channels, permanently removed from the channel allocation pool of frequencies, is wasteful of spectral resources and leads to inefficient system operation. Moreover, dedicated data channel systems lack the capacity to dynamically redistribute or allocate the data traffic load across the available data channels. Such systems typically permanently assign a subscriber unit to a data channel, thereby building in future problems as the number of data subscribers increases on a particular channel.
Placing a fixed timer on the data channel, thereby allotting a fixed amount of time for a data transmission, allows for the data channel to be deallocated (released) after a certain amount of time. The drawbacks of this method are that if the fixed time is too long, the channel will never be released, and if the fixed time is too short, important messages are likely to be truncated or lost.
Accordingly, there exists a need for a trunked communication system that can accommodate both voice and data message formats, and that operates in true trunked manner to efficiently utilize spectral resources.